Thermally Conductive Filler Suspended by Thixotropic Agents in Lubricant Oil

ABSTRACT

Lubricant fluid compositions which include a base oil fluid medium which contains thermally conductive filler particles and a thixotropic shear thinning viscosifier. The shear thinning viscosifier allows the particles to be suspended within the lubricant fluid composition when at rest.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to compositions for lubricant oils which have particular usefulness within an electrical submersible motor.

2. Description of the Related Art

Hydrocarbon and silicone fluids, e.g., oils, can provide electrical isolation between a stator and rotor as well as power leads in an electric motor. Oils also provide lubrication for engines and motors to extend lifetime and prevent failure. Motor oils lubricate surfaces in relative motion and close contact to one another, such as bearings and other metal surfaces, to improve motor efficiency and run life. Oils are also useful for carrying away heat that is generated within the motor, thereby reducing operating temperature. Motor oils of this type are discussed in U.S. Pat. No. 8,076,809 issued to Tingler et al. U.S. Pat. No. 8,076,809 is owned by the assignee of the present invention and is herein incorporated by reference.

A broad range of electrical resistivities, thermal conductivities, and fluid properties exist among oils. Oils are generally selected based upon a desired viscosity at a specified operating temperature. Preferably, oils are selected to ensure proper lubrication or provide centering or lifting pressure as with hydrodynamic bearings such as electric submersible pump (“ESP”) thrust or journal bearings.

Even for electrical devices without moving parts, heat transfer from static components and their electrical isolation are considerations, particularly in high voltage or high current applications. Additional equipment is sometimes needed to aid the cooling of these devices. Thermally conductive fillers are typically not used in fluids such as lubricant oils because of filler sedimentation. The sedimentation is due to the inherent difference between the filler densities (1-5 g/mL) and the fluid medium density (0.7-1.0 g/mL). Settling can be even more pronounced in the absence of perturbation or lack of circulation which occurs during tool cycling.

SUMMARY OF THE INVENTION

The present invention provides lubricant fluid compositions which include a base oil fluid medium which contains thermally conductive filler particles and a shear thinning viscosifier. The inventors have determined that settling can be overcome by adding a shear thinning, thixotropic viscosifier to base oil fluid media that include thermally conductive filler. The viscosifier will cause the lubricant fluid composition to gel and thereby mitigate settling during periods of downtime. However, when the lubricant fluid composition is subjected to flow and shear forces, it thins as a result of the viscosifier. The invention provides lubricant fluid compositions with improved thermal conductivity.

In described embodiments, lubricant fluid compositions include a base oil fluid medium that is used to provide lubrication. The base oil fluid medium may be natural oil, synthetic oil, or a combination including either or both.

In particular embodiments, thermally conductive filler particles are Al₂O₃ (aluminum oxide), SiO₃ silicate), BN (Boron nitride), AlN (Aluminum nitride), and Si₃N₄ (Silicon nitride). In currently preferred embodiments, the typical size of the thermally conductive filler particles is less than 100 μm. Also in currently preferred embodiments, the thermally conductive filler particles are sufficient to raise thermal conductivity of the lubricant fluid composition by more than 5%.

In particular embodiments, thixotropic agents include quaternary amine functionalized clays, substituted styrene acrylate polymers, and polyimide polymers. The thixotropic shear thinning viscosifier causes the lubricant fluid composition to gel when at rest. However, the shear thinning nature of the viscosifier causes the lubricant fluid composition to thin when flowed, such as when a motor containing the lubricant fluid composition is operated. The utility of the viscosifier to gel the lubricant fluid composition substantially prevents settling and sedimentation of the filler particles when the fluid composition is at rest.

According to exemplary embodiments, a lubricant fluid composition in accordance with the present invention includes a fluid medium which contains a thermally conductive filler in a concentration from about 1% to about 20% by volume together with a thixotropic viscosifier agent in a preferred concentration of about 0.25% to about 20% by volume. In further preferred embodiments, the thixotropic agent is present in a concentration that is from about 5% to about 10% by volume. In a particular example, a lubricant fluid composition is provided that includes a base oil fluid medium which contains a thermally conductive filler in the form of BN which is present in the fluid medium at a concentration of about 10% by volume. In addition, the lubricant fluid composition includes a thixotropic viscosifying agent in the form of a solid or liquid that is present in the fluid medium at a concentration of about 0.1% to about 5% by volume.

In other aspects, the invention provides methods for producing hydrocarbon based lubricants having improved thermal, electrical and tribological properties. An exemplary method includes the step of providing a base oil fluid medium and adding a desired amount of thermally conductive filler operable to effectively increase the thermal conductivity of the fluid medium, In addition, the method includes the step of adding a desired amount of a thixotropic viscosifying agent that is effective to suspend the thermally conductive filler particles within the fluid medium.

In particular aspects, the invention provides a submersible electric motor that is centered or lifted during operation by hydrodynamic pressures of the fluid composition in accordance with the present invention. In a described embodiment, a submersible electric motor for downhole use is described, of the type that is used to power a submersible fluid pump for fluid production in a wellbore. An exemplary submersible electric motor in accordance with the present invention includes a shaft and at least one rotor and at least one bearing mounted on the shaft. The electric motor also includes a stator positioned external to the at least one rotor. A running clearance is defined between an internal diameter of the stator and an outside diameter of the rotor, and a lubricant fluid composition is disposed within the running clearance as well as other cavities of the motor. The lubricant fluid composition will have a formulation of the type discussed herein.

BRIEF DESCRIPTION OF THE DRAWINGS

The advantages and other aspects of the invention will be readily appreciated by those of skill in the art and better understood with further reference to the accompanying drawings in which like reference characters designate like or similar elements throughout the several FIGURES of the drawings and wherein:

FIG. 1 is a side, partial cross-sectional view of an exemplary electrical submersible pump.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A lubricant fluid composition of electrically insulating and thermally conductive filler particles in a base oil fluid medium provides improved thermal conduction and electrical insulation to the composition. The base oil fluid medium for the lubricant fluid composition is useful for lubrication in, for example, an electric submersible pump (ESP), electric transformer, motor, electric discharge machining and like devices and environments where electrically insulating fluids and/or thermally conductive fluids are used. Examples of suitable base oil fluid media include natural oil, synthetic oil (e.g., fluorinated oils and silicon-containing oils), or a combination comprising at least one of the foregoing.

As used herein, “natural oil” refers to a naturally occurring liquid or crude oil comprising a mixture of hydrocarbons having various molecular weights, which may have been recovered from a subsurface rock formation, and which may have been subjected to a refining process by distillation or otherwise. As used herein, synthetic oil refers to a hydrocarbon liquid that comprises chemical compounds not originally present in a natural oil, but were instead synthesized from other compounds.

The fluid medium can be any natural oil, various petroleum distillates, or synthetic oil in any rheological form, including liquid oil, grease, gel, oil-soluble polymer composition, or the like, particularly the mineral base stocks or synthetic base stocks used in the lubrication industry, e.g., Group I (solvent refined mineral oils), Group II (hydrocracked mineral oils), Group III (severely hydrocracked oils, sometimes described as synthetic or semi-synthetic oils), Group IV (polyalphaolefins (PAOs)), and Group V (esters (e.g., polyols esters), naphthenes, polyalkylene glycols, silicone oil, fluorinated compounds (e.g., polyhexafluoropropylene oxide, perfluoropolyether (PFPE), perfluoroalkylether (PFAE), and perfluoropolyalkylether (PFPAE)), and the like). Examples include polyalphaolefins, synthetic esters, and polyalkylglycols.

Synthetic oils include hydrocarbon oils and halo-substituted hydrocarbon oils such as polymerized and interpolymerized olefins (e.g., polybutylenes, polypropylenes, propylene-isobutylene copolymers, chlorinated polybutylenes, poly(1-octenes), poly(1-decenes), etc., and mixtures thereof); alkylbenzenes (e.g., dodecylbenzenes, tetradecylbenzenes, dinonylbenzenes, di-(2-ethylhexyl), benzenes, etc.); polyphenyls (e.g., biphenyls, terphenyls, alkylated polyphenyls, etc.); alkylated diphenyl ethers; alkylated diphenyl sulfides; derivatives, analogs and homologs thereof; and the like. Alkylene oxide polymers and interpolymers and derivatives thereof where the terminal hydroxyl groups have been modified by esterification, etherification, etc., constitute another class of synthetic oils. Combinations of the synthetic oils can be used together.

Another suitable class of synthetic oils includes the esters of dicarboxylic acids (e.g., phthalic acid, succinic acid, alkyl succinic acids, alkenyl succinic acids, maleic acid, azelaic acid, suberic acid, sebacic acid, fumaric acid, adipic acid, alkenyl malonic acids, etc.) with a variety of alcohols (e.g., butyl alcohol, hexyl alcohol, dodecyl alcohol, 2-ethylhexyl alcohol, ethylene glycol, diethylene glycol monoalkylethers, propylene glycol, etc.). Specific examples of these esters include dibutyl adipate, di(2-ethylhexyl) sebacate, di-hexyl fumarate, dioctyl sebacate, diisooctyl azelate, diisodecyl azealate, dioctyl phthalate, didecyl phthalate, dicicosyl sebacate, the 2-ethylhexyl diester of linoleic acid dimer, the complex ester formed by reacting one mole of sebacic acid with two moles of tetraethylene glycol and two moles of 2-ethylhexanoic acid, and the like.

Esters useful as synthetic oils also include those made from C5 to C12 monocarboxylic acids and polyols and polyol ethers such as neopentyl glycol, trimethylolpropane, pentaerythritol, dipentaerythritol, tripentaerythritol, etc. Other synthetic oils include liquid esters of phosphorus-containing acids (e.g., tricresyl phosphate, trioctyl phosphate, diethyl ester of decylphosphonic acid, etc.), polymeric tetrahydrofurans and the like.

Lubricant fluid compositions in accordance with the present invention include the base oil fluid medium as well as thermally conductive filler in an amount or concentration that is sufficient to increase the thermal conductivity of the fluid composition. Exemplary thermally conductive filler particles include Al₂O₃ (aluminum oxide), SiO₃ (silicate), BN (Boron nitride), AlN (Aluminum nitride) and Si₃N₄ (Silicon nitride). In preferred embodiments, the thermally conductive filler is added to provide a concentration from about 1% to about 20% by volume relative to the complete lubricant fluid composition.

In accordance with the present invention, the lubricant fluid composition includes a thixotropic agent which acts as a viscosifier to cause the lubricant fluid composition to gel when at rest, thereby largely suspending the thermally conductive filler particles within the lubricant fluid composition rather than allowing them to settle. In particular embodiments, thixotropic agents include organophilic clays and polymers, such as styrene acrylics. An organophilic clay is created by reacting quaternary ammonium compounds with a clay. Clays that can be organophilically modified in this manner include hectorite, bentonite, attapulgite and sepiolite. Organophilic fumed silica is also a possibility. The polymer can be a substituted styrene acrylate copolymer or a polyamide. From the polymeric side, there are a variety of fatty acid polyamides, polyamide waxes and other types of fatty acid derived waxes. The thixotropic agent also has a shear thinning nature which causes the lubricant fluid composition to thin when flowed, such as when a motor containing the lubricant fluid composition is operated. Shear forces imparted to the lubricant fluid composition will thin the fluid and cause it to flow, again suspending the thermally conductive filler particles. In specific embodiments, the thixotropic agent makes up from about 0.25% to about 20% of the lubricant fluid composition by volume. In further preferred embodiments, the thixotropic agent is present in a concentration that is from about 0.1% to about 10% by volume.

In other aspects, the invention provides methods for producing hydrocarbon based lubricants having improved thermal, electrical and tribological properties. An exemplary method includes the step of providing a base oil fluid medium and adding a desired amount of thermally conductive filler operable to effectively increase the thermal conductivity of the fluid medium. In addition, the method includes the step of adding a desired amount of a thixotropic viscosifying agent that is effective to suspend the thermally conductive filler particles within the fluid medium.

According to an embodiment, a lubricant fluid composition described herein is used in a downhole electrical submersible pumping system (ESP) that is disposed in a borehole, wherein the borehole may intersect a subterranean formation. As shown in the FIGURE, the ESP includes at a lower end a submersible electric motor 10, a seal (not shown), and a pump (not shown) on an upper end. The motor 10 and pump are separated by the seal. The motor 10 includes a rotor 20 (or a plurality of rotors 20) and bearings 30 mounted on a motor shaft 40 that is coupled to and drives the pump. The motor shaft 40 is coupled to the pump via a seal section, and the motor shaft 40 is coupled to a shaft in the seal section, which in turn is coupled to a shaft in the pump. The rotor 20 can be a hollow cylinder made of a stack of laminations, a copper bar and end rings, which is supported at each end by the bearings 30. The motor 10 is filled with lubricant fluid composition 50 having a composition as described herein and includes a running clearance 60 located between the internal diameter of the stator 70 and outside diameter of the rotors 20 wherein the lubricant fluid composition 50 provides thermal conduction, electrical insulation, and lubrication for items such as the bearings 30 in order to carry away (dissipate) heat generated by friction from the rotor 20 and windage losses while being an electrical insulator between the stator 70 and the rotor 20. The lubricant fluid composition 50 within the running clearance 60 can be circulated within the motor 10 through a hole 80 in the shaft 40. The lubricant fluid composition 50 in the motor can also be used in the seal and communicates and circulates between the seal and motor 10. The lubricant fluid composition 50 used in the seal assists with the cooling of a bearing (e.g., a thrust bearing) in the seal. Thus, another aspect of the present invention provides methods of lubricating an electric motor 10 of an electric submersible pump assembly.

Those of skill in the art will recognize that numerous modifications and changes may be made to the exemplary designs and embodiments described herein and that the invention is limited only by the claims that follow and any equivalents thereof. 

What is claimed is:
 1. A lubricant fluid composition comprising: a base oil fluid medium; a thermally conductive filler within the fluid medium; and a thixotropic shear thinning viscosifier within the fluid medium.
 2. The lubricant fluid composition of claim 1 wherein the base oil fluid medium comprises at least one of the group consisting of natural oil, synthetic oil or a combination comprising at least one of the foregoing.
 3. The lubricant fluid composition of claim 1 wherein the thermally conductive filler comprises at least one of the group consisting of Al₂O₃ (aluminum oxide), SiO₃ (silicate), BN (Boron nitride), AlN (Aluminum nitride), Si₃N₄ (Silicon nitride) or a combination comprising at least one of the foregoing.
 4. The lubricant fluid composition of claim 1 wherein the thixotropic shear thinning viscosifier comprises at least one of the group consisting of organophilic clays, and polymers, including substituted styrene acrylic copolymers, a polyamide, or a combination comprising at least one of the foregoing.
 5. The lubricant fluid composition of claim 3 wherein there is a concentration of from about 1% to about 20% by volume of thermally conductive filler.
 6. The lubricant fluid composition of claim 4 wherein there is a concentration of from about 0.1% to about 10% by volume of thixotropic shear thinning viscosifier.
 7. A method of making a lubricant fluid composition comprising the steps of: providing a base oil fluid medium; adding a thermally conductive filler to the base oil fluid medium in an amount effective to increase the thermal conductivity of the base oil fluid medium; and adding a thixotropic viscosifier agent to the base oil medium in an amount effective to suspend particles of the thermally conductive filler within the base oil fluid medium when the lubricant fluid composition is substantially at rest.
 8. The method of claim 7 wherein the step of adding a thermally conductive filler further comprises adding the thermally conductive filler in a concentration of from about 1% to about 20% of the lubricant fluid composition by volume.
 9. The method of claim 7 wherein the step of adding a thixotropic viscosifier agent further comprises adding the thixotropic viscosifier agent in a concentration of from about 0.1% to about 10% of the lubricant fluid composition by volume.
 10. The method of claim 7 wherein the base oil fluid medium comprises at least one of the group consisting of natural oil, synthetic oil or a combination comprising at least one of the foregoing.
 11. The method of claim 7 wherein the thermally conductive filler comprises at least one of the group consisting of Al₂O₃ (aluminum oxide), SiO₃ (silicate), BN (Boron nitride), AlN (Aluminum nitride), Si₃N₄ (Silicon nitride) or a combination comprising at least one of the foregoing.
 12. The method of claim 7 wherein the thixotropic shear thinning viscosifier comprises at least one of the group consisting of organophilic clays and polymers, such as styrene acrylics or a combination comprising at least one of the foregoing.
 13. A submersible electric motor for downhole use, the motor comprising: a shaft; at least one rotor and at least one bearing mounted on said shaft; a stator positioned external to the at least one rotor; a running clearance located between an internal diameter of the stator and an outside diameter of the rotor; and a lubricant fluid composition disposed within the running clearance and other cavities of the motor; wherein the lubricant fluid composition comprises a base oil fluid medium, a thermally conductive filler, and a thixotropic viscosifier.
 14. The submersible electric motor of claim 13 wherein the fluid medium comprises at least one of the group consisting of natural oil, synthetic oil or a combination comprising at least one of the foregoing.
 15. The submersible electric motor of claim 13 wherein the thermally conductive filler comprises at least one of the group consisting of Al₂O₃ (aluminum oxide), SiO₃ (silicate), BN (Boron nitride), AlN (Aluminum nitride), Si₃N₄ (Silicon nitride) or a combination comprising at least one of the foregoing.
 16. The submersible electric motor of claim 13 wherein the thixotropic viscosifier comprises at least one of the group consisting of organophilic clays and polymers, such as styrene acrylics or a combination comprising at least one of the foregoing. 